The present invention relates to a process for removing sulphur oxides, optionally together with other acid gases such as, in particular carbon dioxide and nitrogen oxides, from a fluid stream.
Combustion of fossil fuels generates the acid gases, carbon dioxide (CO2), sulphur oxides (SOx), and nitrogen oxides (NOx). Emission of acid gases into the air is considered to be the main cause of global effects on the environment which are termed the greenhouse effect. The removal of acid gases from combustion exhaust gases is therefore desirable to reduce the environmental effects of these sources. Carbon dioxide capture from combustion exhaust gases may also provide supplies for CO2 enhanced oil recovery (EOR).
Sulphur oxides, besides other acid gases, may also be present in refinery off-gases. The tail gas from Claus processes, while small in volume, has a relatively high concentration of sulphur oxides. These gases cannot be vented and still meet environmental regulatory requirements.
Current aqueous scrubbing technologies remove acid gases by treating the fluid streams with aqueous solutions of inorganic or organic bases, e.g. alkanolamines, as absorbents. On the dissolution of acid gases, ionic products form from the base and the acid gas components. The absorbent can be regenerated by heating, expansion to a lower pressure or stripping, in which case the ionic products react back to acid gases and/or the acid gases are stripped off by means of steam. After the regeneration process the absorption liquid can be reused.
Non-aqueous absorption liquids have been used to remove acid gases from fluid streams. It is claimed that non-aqueous absorption liquids require less energy for regeneration than aqueous solutions of organic bases. Single-component alcoholic physisorption solvents such as RECTISOL® and SELEXOL® are commercially available for CO2 separation but perform poorly in the humid, near-ambient pressure conditions associated with combustion exhaust gas.
Ionic liquids are another non-aqueous absorption liquid currently being developed. These absorption liquids have higher CO2 loadings than some aqueous amines, and are regenerable under milder conditions.
WO 2009/097317 discloses reversible acid-gas binding liquid systems that permit capture of acid gases and release of the acid gases from the liquid by the activation of a trigger. The exemplified systems are equimolar mixtures of amidine or guanidine nitrogen bases and alcohols. They are non-ionic room temperature liquids that react with CO2 to form room-temperature ionic liquids. CO2 is captured via the formation of amidinium and guanidinium alkyl carbonate salts derived from the conjugate bases of the deprotonated alcohol components.